The present invention is the field of ripple control. It is known that this is a system for transmitting switching commands via the existing distribution systems of the electrical supply which makes it possible to influence the system load directly from a central point in the case of load peaks or production bottlenecks.
The transmitter which transmits the audio-frequency switching signals into the system to be controlled assumes a central position in a ripple control system. A command device transmits to the ripple control transmitter the bit pattern of the required transmission and the transmitter converts this pattern into a signal with the appropriate ripple control frequency which is fed into the system through a coupling arrangement. Each connected ripple control receiver receives the signal, decodes it and appropriately switches a particular connected load.
For purposes of considering its operation, the ripple control transmitter can be divided into a control section and a power section. The control section contains all circuits required for control and supervision, while the power section contains, among other things, a control and measurement converter, a rectifier, an inverter and an output section. Reference input variables trigger in the control section control signals which generate a load voltage having the required ripple control frequency from a rectified voltage in the inverter branches in the power section.
The power section has hitherto been implemented in the form of conventional thyristor converters or, even earlier, as a motor converter. From the ample literature in this field, the following should be mentioned here by way of example: M. Meyer "Self-commutated thyristor inverters", Siemens reference book, 3rd edition 1974; B. D. Bedford, R. G. Hoft "Principles of Inverter Circuits", John Wiley, New York 1964.
Motor converters are expensive and require intensive servicing and are subject to a mechanical wear phenomena. In addition, their frequency is generally not constant. They require a longer start-up phase and produce noise and vibrations.
Conventional thyristor converters require complicated commutation devices since the thyristors cannot be turned off via the gate electrode. Such commutating devices are expensive, voluminous and heavy and must be designed for a narrow-band frequency. The required reactor coils produce noise and the commutation only operates within a limited range of load angles. In addition, the efficiency is highly frequency- and load-dependent and, as a rule, is less than 85%.
For some years, higher-power turn-off thyristors, so-called GTO thyristors (GTO=Gate Turn-Off) have been available for use in current converters as a result of which the elaborate commutation elements are no longer needed in the inverter section of the current converter. They are replaced by a thyristor turn-on and -off device (gate driver) by means of which the thyristors can be turned on and off via the gate. This increases, on the one hand, the efficiency and, on the other hand, the system becomes virtually noiseless, independent of load angle, free in the frequency range, smaller and lighter.
Previous main fields of application of GTO thyristors are converters for industrial drives and control systems, inverters in auxiliary current supply systems for rail and road traffic, inverters for three-phase drives for rail vehicles and uninterruptable power supply systems. In the application for motor drives, the cycle frequency range needed is usually some 100 Hz; previous gate drivers are operated at a frequency of about 3000 Hz at a maximum.
However, ripple control transmitters require a much higher frequency range since they must cover a frequency range of approximately 100-2000 Hz. By using 6-pulse inverter circuits with an inverter output current, the current/time integral of each phase of which is equal to zero for each transmitting current period, the sum of all three output currents also being equal to zero at any time, a cycle frequency corresponding to six times the ripple control frequency is needed. For a 2000 Hz ripple control frequency, this means a cycle frequency of 12000 Hz for the GTO thyristor and the gate driver which is a multiple of the previously possible maximum frequency.